1. Field of the Invention
The present invention relates to a magnetic disk capable of achieving a high density magnetic recording and to a magnetic disk apparatus having the magnetic disk mounted thereto, particularly, to a magnetic disk capable of reading-writing with the fly height of the magnetic head set at a level not higher than 10.0 nm and to a magnetic disk apparatus equipped with the particular magnetic disk.
2. Description of the Related Art
In general, a magnetic disk apparatus comprises a magnetic disk arranged within a case, a spindle motor for supporting and rotating the magnetic disk, and a head suspension assembly (hereinafter referred to as HSA) including a magnetic head for reading-writing information in and from the magnetic disk.
The HSA includes a slider having a magnetic head formed thereon, a suspension supporting the slider, and an arm supporting the suspension. The HSA is rotatably supported by a bearing assembly. The magnetic head can be moved to an optional position on the magnetic disk by allowing the HSA to be rotated by a voice coil motor.
In order to suppress the change in the slider behavior when the magnetic disk is unintentionally brought into contact with the magnetic disk in the magnetic disk apparatus, it is necessary to increase the roughness of the magnetic disk surface to some extent so as to reduce the adhesion force of the slider relative to the magnetic disk. In the conventional magnetic disk apparatus, the fly height of the magnetic head is not smaller than 10.0 nm. Therefore, even in the case of using a magnetic disk in which the height of the asperity on the surface of the disk (i.e., the asperity height) is not smaller than 4.0 nm, it is possible to ensure the fly height margin of the magnetic head, i.e., the minimum distance between the magnetic head and the magnetic disk, so as to make it possible to maintain the fly height stability of the magnetic head.
However, the recording density of the magnetic disk has been increased to reach 70 GB/inch2 in recent years so as to make it necessary to set the fly height of the magnetic head to a level not higher than 10.0 nm. Under the particular situation, the fly height margin of the magnetic head is eliminated if the asperity height of the magnetic disk surface is not smaller than 4.0 nm. As a result, the thermal asperity is generated, or the reliability is lowered. Incidentally, thermal asperity noted above denotes the phenomenon that the output of the MR (magnetoresistance sensor) in the magnetic head is changed by the heat that is generated when the magnetic head collides against the asperity.
In order to improve the SN ratio included in the electromagnetic characteristics, it is attempted in recent years to apply a texture processing to the substrate of the magnetic disk so as to impart a magnetic anisotropy to the magnetic disk. To be more specific, the easy axis of magnetization of a Co alloy layer constituting the magnetic recording layer is oriented in the circumferential direction by the texture processing, with the result that the residual magnetization and the squareness ratio in the circumferential direction are rendered high relative to those in the radial direction of the magnetic disk. Since the read output is improved substantially in proportion to the residual magnetization, the thickness of the magnetic layer can be decreased so as to improve the magnetization transition width, the capability of suppressing the noise and the overwrite characteristics. It should be noted that the texture medium (anisotropic medium) having the texture processing applied thereto exhibits an improvement in the resolution, the full width at half-maximum, and the SN ratio and, thus, is allowed to have a big merit as a recording medium having a high recording density. Such being the situation, the practical use of the texture medium constitutes an effective means for achieving a high density recording.
However, the asperities on the disk surface are also removed by the texture processing, with the result that the surface roughness of the magnetic disk tends to be rendered very low. It follows that the absorption properties of the magnetic head relative to the magnetic disk are increased. Such being the situation, in order to obtain a texture medium having a high reliability, it is necessary to control severely the surface roughness of the disk so as to optimize the surface roughness.
An average roughness Ra, which is defined in Japanese Industrial Standards (JIS B 0601) is widely used as an index of the surface roughness. In the average roughness Ra noted above, the depth or height of the asperity or valley on the surface of the magnetic disk from the center line of the surface height is integrated and averaged. However, the friction force that is generated when the magnetic head is brought into contact with the magnetic disk is much related to the contact area occupied by the asperity of the magnetic disk and is less related to the valley. Such being the situation, the average roughness Ra is insufficient for use as an index of the surface roughness denoting the relevancy to the fly height stability of the magnetic head.
Difference Rp between the center line of the height of the asperity on the surface of the recording and the maximum height of the asperity on the surface of the magnetic disk is also known as another index. However, the difference Rp noted above does not denote the average height of the asperity. It follows that, even if the index Rp is large, it is possible for the surface having a low average roughness such as a texture medium to fail to lower the friction force in the stage of the contact between the magnetic head and the asperity on the surface of the magnetic disk unless the high asperity are included in a large area on the surface of the recording medium. It follows that the relevancy of the index Rp to the fly height stability is low.
In recent years, the difference ΔBH[0.01, 50] between the height BH[0.01%] at which the contact ratio in the bearing curve is 0.01% and the height BH[50%] at which the contact ratio in the bearing curve is 50%, i.e., ΔBH[0.01, 50]=|BH[0.01%]−BH[50%]|, is used as an index of the surface roughness, as disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 2001-160214. Proposed in this prior art is a magnetic disk having the difference ΔBH[0.01, 50] set to fall within a range of between 0.3 nm and 6.0 nm.
Proposed in Japanese Patent Disclosure
No. 2001-143246 is a substrate for an information recording medium in which the height of the surface roughness at 0.4% of the contact ratio falls within a range of between 2.0 nm and 7.0 nm under the condition that the case where the contact ratio of the surface roughness is 50% is used as the standard height.
However, where these technologies are applied to the texture medium, the index of the roughness noted above is insufficient so as to make it difficult to achieve both a high recording density and a high reliability.
To be more specific, in the magnetic disk noted above, the roughness is defined with the height at 50% of the area ratio of the bearing curve used as a standard. However, the roughness is defined with attentions paid to the height of a small number of asperities having the contact area ratio of 0.01% or 0.4%, particularly, to the high asperities having a height not smaller than 5.0 nm. In the texture medium, however, the maximum height of the asperity is not larger than 3.0 nm. It is considered reasonable to understand that the asperities on the surface of the magnetic head are pushed into the head slider by several nanometers when the head slider is brought into contact with the surface of the magnetic disk. Therefore, the factor substantially determining the adhesion properties between the slider and the surface of the magnetic disk is the contact area at the time when the asperities are pushed into the slider in a height of up to about 1.0 nm. It follows that it is insignificant to argue the high asperities having a height not smaller than 5.0 nm and, thus, the index of the roughness noted above is insufficient in respect of the texture medium.
The magnetic disk disclosed in Japanese Patent Disclosure No. 2001-160214 noted above covers the case where the asperity has a height of 4.0 to 6.0 nm. However, in order to realize a desired fly height of the magnetic disk, which is not larger than 10.0 nm, and a desired recording density, which is not lower than 70 GB/inch2, it is difficult for the asperity height noted above to achieve both a high recording density and a high reliability.
In the magnetic disk disclosed in Japanese Patent Disclosure No. 2001-143246 noted above, the surface roughness that is important in realizing the recording with a very low fly height of the magnetic head is not substantially defined.